The Impact of the Ge Concentration in the Source for Vertical Tunnel-FETs

The Tunnel Field Effect Transistors (TFETs) are emerging as one of the post-CMOS alternatives for future technologies. Since the drain current of these devices is predominantly based on the band-to-band tunneling (BTBT) mechanism, the subthreshold swing may reach values smaller than 60mV/dec (1). However, for silicon devices, a small current drive has been obtained. In order to improve the on-state current, devices with different materials and architectures have been studied (2,3). In this work, the impact of the germanium amount in the source of a silicon nanowire (NW) TFET is analyzed.

     All analyzed vertical nanowire TFETs were fabricated at imec/Belgium with the following characteristics: gate stack composed by a dielectric of 3nm HfO₂ on top of 1nm interfacial SiO₂ covered by a TiN metal and amorphous silicon layer. Four different source compositions are studied: Si(100%), Si_0.73Ge_0.27, Si_0.56Ge_0.44and Ge(100%). More process details can be found in (2,3). Each device used in this work is composed of 100 nanowires in parallel. A schematical view of one nanowire can be seen in figure 1.

     The impact of the source composition on the TFET behavior can be evaluated in figure 2. By increasing the germanium amount of the source, the bandgap decreases which results in an increased BTBT generation rate and a higher on-state current (I_ON) without degrading I_OFF, resulting in a better subthreshold swing.

     Figure 3 shows the influence of %Ge on the NW-TFET transconductance (gm) and output conductance (g_D). As can be observed in figure 3, when %Ge increases, gm also increases thanks to the higher tunneling current at the source side. However, the g_D becomes worse due to the higher influence of the drain electric field on the drain current for devices dominated by a higher BTBT current (4).

     The intrinsic voltage gain (A_V=gm/g_D) presents an optimum point around 27% of Ge due to a competition between the gm and g_D behavior. From 0 to 27% of Ge, the gm is the predominant parameter thanks to the increase of BTBT current. For higher %Ge, the drain electric field is the dominant g_D degradation factor, since the devices are operating at a higher level of the BTBT component (4).

     In order to better understand the effect of the Ge amount at the TFET source, the low-frequency noise spectra of Si_1−xGe_x devices were analyzed. The lowest current noise PSD was obtained for the Si source sample (about one order of magnitude lower 1/f noise at 10 Hz). This suggests that the presence of Ge at the interface with the gate dielectric may be responsible for this observation. Similar results have already been observed for Si_1−xGe_x channel MOSFETs, where the 1/f noise increases with Ge content, resulting in an increase of traps and defects at the interface and in the dielectric layer (5,6).

    A more detailed analysis about TFETs with different Ge amount at the source terminal will be presented at the conference.

1. W. Y. Choi et al., IEEE Electron Device Lett. 28, 743 (2007).

2. R. Rooyackers et al., IEDM, 92 (2013).

3. Vandooren, A. Solid State Electronics, Vol. 83, p.50 (2013).

4. P. G. D. Agopian et al., IEEE Trans. Electron Device, DOI: 10.1109/TED.2014.2367659, (2014).

5. A. Tanabe et al.,Proc. Electrochem. Soc., 2004, pp. 483–492.

6. K. Fobelets et al., J. Appl. Phys., vol. 103, no. 4, pp. 044501-1–044501-4, 2008.